The present invention relates to a copolyester with improved physical properties, particularly excelling in toughness, which is suited for molding.
The invention further relates to a resin composition comprising said copolyester, which is excellent in toughness, impact resistance and other mechanical properties, and moldability characteristics such as injection moldability.
Polyethylene terephthalate (hereinafter sometimes referred to briefly as PET) has been used in various applications. While this resin has the advantage of having a high secondary transition point and high melting point as compared with polybutylene terephthalate, which is also a thermoplastic polyester, it also has the disadvantage that when fully crystallized with its molecular chains a non-oriented state, it is brittle and unless reinforced with a reinforcing material such as glass fiber, shaped articles manufactured by injection molding or extrusion molding are not of practical value. To impart ductility of polyethylene terephthalate, there is the method comprising the addition of a metallic ion neutralisate of a copolymer of an .alpha.-olefin with an .alpha.,.beta.-ethylenically unsaturated carboxylic acid (Japanese Patent Laid-open No. 52-84244) but the melt viscosity of the composition is markedly increased in this method so that trouble is encountered particularly in injection molding. Furthermore, the composition containing such a copolymer exhibits of discoloration on heating.
On the other hand, as a composition having sufficient toughness, a glass fiber-reinforced polybutylene terephthalate composition is known but this composition has the disadvantage of a low heat distortion temperature.
PET whether reinforced or not with glass fiber or other filler material does not possess a sufficiently high impact resistance and the problem of breakage of shaped articles in secondary processing and during transport is frequently encountered.
The general method for improving the impact resistance of PET, whether reinforced or not, comprises adding a certain elastomeric polymer to the PET.
For example, Japanese Patent Publication No. 45-26223 discloses a copolymer of an .alpha.-olefin with a saturated aliphatic monocarboxylic acid vinyl ester as an impact strength improving agent for polyester resin. Japanese Patent Publication No. 45-26224 describes a copolymer of an acrylic ester with a conjugated diene as an impact strength improving agent for polyester resin. In Japanese Patent Publication No. 45-26225, there is disclosed an ionomer as an impact strength improving agent. However, shaped articles manufactured by the above-mentioned methods are still not fully satisfactory in impact strength.
Several other methods are known for improving the impact strength of unreinforced PET or reinforced PET. By way of example, Japanese Patent Laid-open No. 51-144452, No. 52-32045 and No. 53-117049 teach the technique of blending a polyester resin with a copolymer of an .alpha.-olefin with an .alpha.,.beta.-unsaturated carboxylic acid glycidyl ester. The technique of using an ethylene copolymer as a third component in addition to the above-mentioned copolymer is disclosed in Japanese Patent Laid-open No. 58-17148 and No. 58-17151, and the technique of using polyphenylene sulfide as an additive component is described in Japanese Patent Laid-open No. 57-92044.
However, even these techniques are not fully capable of assuring sufficient impact strength.
Of all the plastics, aromatic polycarbonate is known to be the resin having the greatest impact resistance, and there was an early attempt to blend PET with this resin for improving the impact resistance of PET (Japanese Patent Publication No. 36-14035). More recently, U.S. Pat. No. 4,257,937, for instance, teaches the combination of a polyacrylate resin and an aromatic polycarbonate resin as an impact strength improving agent for polyester resin. By this method, a fairly high impact resistance has been obtained. Japanese Patent Laid-open No. 59-161460 shows that in improving the impact strength of PET with a polyacrylate resin and an aromatic polycarbonate resin, the concomitant use of poly(1,4-butylene terephthalate) results in a further remarkable improvement in impact strength. However, even by this method, the impact strength (Izod impact strength) of the product is simply approaching that of a PBT/ polyacrylate/ aromatic polycarbonate resin composition and does no exceed it.
The present inventors previously proposed the use of a PBT polyester in a predetermined proportion in combination with a PET polyester and disclosed that by modifying the blend with a metal salt of a certain carboxyl group-containing polymer, a shaped article with a very high impact strength can be obtained.
However, since this composition is prepared by blending the PET polyester with the PBT polyester at a high temperature, transesterification is liable to take place. Moreover, when molding scraps are pulverized and the resulting pellets are added to the molding material for reuse of the raw material, the high impact strength attainable with the virgin molding material cannot be obtained. Moreover, the composition has a further disadvantage in that the shaped article is either poor in surface gloss or not satisfactory in mold releasability.
Further, the conventional composition prepared by adding an elastomeric polymer to a polyester resin showed a decrease in impact strength when the shaped article was exposed to a high temperature for a long time.
On the other hand, it is known that the impact strength of a filler-reinforced polyester resin can be improved by blending a very large amount of an elastomer therewith (e.g. Japanese Patent Publication No. 59-30742). However, the incorporation of such a large amount of the elastomer results in the deterioration of the inherent heat resistance and mechanical strength of the polyester.
Japanese Patent Laid-open No. 53-102360 teaches that the addition of PBT to a filler-reinforced PET composition results in an improved resistance to warping. However, this blending does not provide for an increased impact strength.
The present inventors previously showed that the use of a PBT type polyester in a specified proportion in combination with a PET type polyester and the modification of the blend with a specified amount of a metal salt of a carboxyl group-containing polymer results in a very high product impact strength, with a notched Izod impact strength value exceeding 20 kg.cm/cm being sometimes obtained. However, since this composition is prepared by blending a PET type polyester with a PBT type polyester at a high temperature, transesterification takes place so that the heat resistance of the shaped article is poor. Moreover, as mentioned above, when molding scraps are pulverized and added to the virgin molding material for reclamation and reuse of the scraps, the high impact resistance achieved with the virgin material cannot be obtained.
The conventional glass fiber-reinforced polyethylene terephthalate referred to above has the disadvantage that its impact resistance decreases on prolonged exposure to high temperature and that satisfactory surface gloss and mold releasability cannot be achieved unless the mold temperature is set at a comparatively high level.